This invention relates to apparatus for conditioning granular material, especially sand for reclamation.
Reconditioning of used sand, particularly foundry sand, involves reducing used core elements and molded sand lumps into individual grains. The quality of reclaimed sand must be comparable to that of new sand so that it may be satisfactorily substituted for new sand in core and molding sand mixtures. Thus, it is usually necessary to remove accumulated coatings from around individual sand grains. The accumulated coatings may be inorganic, such as clay minerals for grain molding sand, organics such as pitch, cereals and resins, or combinations such as clay used in conjunction with seal coal, pitch and cereals.
Reclamation systems of the prior art have generally crushed lumps of molded sand in a hammermill, ring crusher, jaw or roll crusher. The coatings on the sand grains are then removed by one of several methods known in the prior art; i.e., the wet method, the thermal method and the pneumatic method. The wet method of reclamation consists of mixing the sand with water with a mechanical scrubbing action. A considerable portion of the coating on the sand grains may be removed depending upon the efficiency of the scrubbing and the type of coating. In the thermal method of reclamation, the sand is heated at a temperature of about 1200.degree. F. to about 1500.degree. F. This treatment completely removes the carbonaceous matter. In the pneumatic method of reclamation, pneumatic sand scrubbers utilize low pressure air to make used sand grains sandblast each other. The disadvantages of this latter system are the high power requirements necessary to generate the high velocity air stream and the necessity for having the sand in a free-flow condition. To a certain extent, these disadvantages have been corrected by methods described in a British patent specification No. 1,195,226. A method for removing coatings on sand is described wherein the sand is thrown against a target by a mechanical throwing means with sufficient force to free contaminants from the particles of sand. However, this latter system does not effectively minimize power requirements or achieve adequate removal of contaminants from the conditioned particles.
U.S. Pat. No. 3,782,643, issued Jan. 1, 1974, and U.S. Pat. No. 3,881,664, issued May 6, 1975, are representative of state-of-the-art sand conditioning apparatus of the type having a centrifugal throwing wheel for projecting a stream of material against an impact surface with air stream separation and removal of fines and contaminants from the sand particles. Sand conditioning apparatus of this type has been sold and successfully operated for about ten years. In recent years, howeveer, there has been an increased requirement for control of the amount of fines remaining in the mix after completion of the scrubbing cycles. The apparatus of the current design has not, in all cases, controlled the fines to the extent required. In some cases, additional equipment has been used for further treatment of reclaimed sand with the sand first fed into an elevator and then up into an air wash separator such as that shown in U.S. Pat. No. 3,368,677. Further, excessive wear of certain parts of the apparatus subject to erosive conditions due to impacting sand has been noted.
It will be understood that it would be highly desirable to improve the operation of the present conditioning apparatus of the centrifugal throwing wheel type to gain improved fines control and so eliminate the need for additional equipment and processing to control fines. If excessive wear of parts of the apparatus can be reduced, increased acceptance of the apparatus is likely.